Climate change is likely to present opportunities for some sectors and regions. For example, agriculture could expand into regions where it currently is limited by low temperatures, if adequate soils are present (see Chapter 5). Thinning of Arctic sea ice might allow surface navigation in areas that previously were accessible only to submarines and icebreakers (see Chapter 16). The increase in winter temperature could decrease heating demand or mortality from cold spells (see Chapter 9). However, climate change also is likely to have numerous negative effects on human development and well-being. This is documented in Chapers 4-17 and reflected in the first sentence of the UNFCCC, which states that "changes in the Earth's climate and its adverse effects are a common concern of humankind" (United Nations, 1992). The very existence of the UNFCCC demonstrates that the international community exhibits great concern for the risks that climatic change represents for human development and well-being, despite the potential opportunities it offers. Those risks are classified in Article 2 of the UNFCCC, which describes the Convention's ultimate objective (preventing "dangerous anthropogenic interference with the climate system"). That article mentions the need "to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened, and to enable economic development to proceed in a sustainable manner." Because a key function of the IPCC's assessment reports is to help decisionmakers determine what constitutes "dangerous anthropogenic interference with the climate system" (as evidenced by approval at a 1999 IPCC Panel meeting of a series of Policy-Related Questions directed to the Synthesis Report authors; IPCC, 1999b), the discussion that follows is framed according to the aforementioned three categories of impacts.
The speed and magnitude of climate change affect the success of species, population, and community adaptation. The rate of climatic warming may exceed the rate of shifts in certain species ranges; these species could be seriously affected or even disappear because they are unable to adapt (Chapter 5). Some plant and animal species (such as endangered species generally and species adapted to narrow niches for which habitat is discontinuous and barriers impede or block migration) and natural systems (such as coral reefs, mangroves, and other coastal wetlands; prairie wetlands; remnant native grasslands; montane ecosystems near ridges and mountaintops; and ecosystems overlying permafrost) could be adversely affected by regional climatic variations that correspond to a less than 1°C mean global warming by 2100. With mean warming of 1-2°C by 2100, some regional changes would be significant enough so that adverse impacts to some of these highly sensitive species and systems would become more severe and increase the risk of irreversible damage or loss, and additional species and systems would begin to be adversely impacted. Warming beyond 2°C would further compound the risks (note discussions and citations in Chapters 5 and 19).
Human production factors notwithstanding, food production is influenced mostly by the availability of water and nutrients, as well as by temperature. Increases in temperature could open new areas to cultivation, but they also could increase the risk of heat or drought stress in other areas. Livestock (e.g., cattle, swine, and poultry) are all susceptible to heat stress and drought (Gates, 1993). The effects of climatic changes-even smooth trends-will not be uniform in space or time. For smoothly evolving climatic scenarios, recent literature (see Chapter 5) tends to project that high latitudes may experience increases in productivity for global warming up to a 1°C increase, depending on crop type, growing season, changes in temperature regimes, and seasonality of precipitation. In the tropics and subtropics-where some crops already are near their maximum temperature tolerance and where dryland, nonirrigated agriculture predominates-the literature suggests that yields will tend to decrease with even nominal amounts of climate change (IPCC, 1998; Chapter 5). Moreover, the adaptive capacity of less-developed countries in the tropics is limited by financial and technological constraints that are not equally applicable to more temperate, developed countries. This would increase the disparity in food production between developed and developing countries. For global warming greater than 2.5°C, Chapter 5 reports that most studies agree that world food prices-a key indicator of overall agricultural vulnerability-would increase. Much of the literature suggests that productivity increases in middle to high latitudes will diminish, and yield decreases in the tropics and subtropics are expected to be more severe (Chapters 5 and 19). These projections are likely to be underestimates, and our confidence in them cannot be high because they are based on scenarios in which significant changes in extreme events such as droughts and floods are not fully considered or for which rapid nonlinear climatic changes have not been assumed (Section 2.3.4 notes that vulnerability to extreme events generally is higher than vulnerability to changing mean conditions).
Water availability can be regarded as another component of food security. Water quantity and distribution depends to a large extent on rainfall and evaporation, which are both affected in a changing climate. Typically, estimated patterns of changes for 2100 under SRES scenarios include rainfall increases in high latitudes and some equatorial regions and decreases in many mid-latitude, subtropical, and semi-arid regions-which would increase water stress (the ratio between water usage and renewable flow) in the latter regions and decrease it in the former. As noted in Chapter 4, negative trends in water availability have the potential to induce conflict between different users (e.g., Kennedy et al., 1998). For perspective, it should be remembered that the capability of current water supply systems and their ability to respond to changes in water demand determine to a large extent the severity of possible climate change impacts on water supply. Currently, 1.3 billion people do not have access to adequate supplies of safe water, and 2 billion people do not have access to adequate sanitation (Gleick, 1998; UNDP, 1999). In addition to changes in average water supply, climate extremes such as droughts and floods often are projected to become a larger problem in many temperate and humid regions (IPCC, 1998; Table SPM-1, WGII TAR Summary for Policymakers).
Sustainable development, as noted earlier, implies "meeting the needs of the present without compromising the ability of future generations to meet their own needs" (WCED, 1987). Besides food and water, essential needs include a space to live, good health, respite from extreme events, peace and basic freedom, energy and natural resources that allow development, and so forth. Each of these factors could be affected by climatic change. For example, if there were no significant adaptive responses, a 1-m sea-level rise would decrease the area of Bangladesh by 17.5% or that of the Majuro Atoll in the Marshall Islands by 80%. Human health impacts of global climatic change include changes in the geographic range and seasonality of various infectious diseases (with positive and negative impacts), increases in mortality and morbidity associated with heat waves, and effects on malnutrition and starvation in some regions as a result of redistribution of food and water resources (Chapter 9). The possibility of improved conditions in other regions remains, but, as noted in Chapter 5, the literature tends to project that positive effects in agriculture would be concentrated in high latitudes and negative effects in lower latitudes-precisely where problems of hunger already exist. The frequency and severity of extreme events such as heat waves, high rainfall intensity events, summer droughts, tropical cyclones, windstorms, storm surges, and possibly El Niño-like conditions are likely to increase in a warmer world (Table SPM-1, WGII TAR Summary for Policymakers), which would have a range of adverse impacts and would affect the conditions of development. Migration of populations affected by extreme events or average changes in the distribution of resources might increase the risks of political instabilities and conflicts (e.g., Myers, 1993; Kennedy et al., 1998; Rahman, 1999). For each of these potential impacts, the relative vulnerability of different regions to adverse impacts of climatic change is largely determined by their access to resources, information, and technology and by the stability and effectiveness of their institutions. This means that possibilities to promote sustainable development will be affected more negatively by climatic change in developing countries and among less-privileged populations. Thus, climatic change could make satisfying the essential needs of these populations more difficult, in the short term and in the long term. In that sense, climatic change is likely to increase world and country-scale inequity, within the present generation and between present and future generations, particularly in developing countries. Given this potential vulnerability, steps to strengthen adaptive and mitigative (see TAR WGIII Chapter 1) capacity and to lessen nonclimatic stressors could well enhance sustainable development.
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